Light emitting diodes (LEDs) using nitride semiconductor materials have low toxicity, and characteristically have high efficiencies and long lives. Such LEDs are used in display devices, signals, and lightings, and are available as products in the market. Laser diodes (LDs) using nitride semiconductor materials are used in light sources for performing writing on and reading from high-density memory disks, and are available as products in the market.
By applying a multiquantum well (hereinafter also referred to as MQW) structure of InxGa1-xN to an active layer of a nitride semiconductor, a high-efficiency light emitting device having a high luminance can be formed. In a typical quantum well structure, a 1.5 nm to 5 nm material mainly containing InxGa1-xN is used for the quantum well layer, and GaN is mainly used for the barrier layer, to achieve high-luminance light emission. Particularly, the luminous efficiency and emission wavelength can be varied by changing the composition ratio x of In as a group III element forming the quantum well layer and changing the thickness of the quantum well layer. For example, by increasing the composition ratio x of In in the quantum well layer, light emission of longer wavelengths can be achieved.
To form the above described MQW, it is necessary to successively form InxGa1-xN layers containing different amounts of In from one another. To grow GaN by conventional metal organic vapor phase epitaxy (MOVPE) under optimum conditions, the growth temperature should be in the range of 1000° C. to 1200° C., and a H2 gas should be used as the carrier gas. Meanwhile, to grow InN under optimum conditions, the temperature should be in the range of 500° C. to 650° C., and a N2 gas should used as the carrier gas. There are great differences in growth conditions between the two, and there are many problems in the crystal growth of InxGa1-xN mixed crystals. For example, since the optimum growth temperature of InN is much lower than the optimum growth temperature of GaN, an even lower temperature is required in the crystal growth of an InxGa1-xN layer having a higher In composition ratio. In a case where a GaN layer to be a quantum well layer requiring a high temperature is successively grown after an InxGa1-xN layer to be a quantum well layer such a low temperature is grown, many pits (holes) and clusters (also called protrusions or inclusions) are formed in the surface, and many defects are induced. As a result, a high-efficiency light emitting device having a high luminance cannot be obtained. This problem becomes even more serious in cases where an even lower temperature is required for the growth or where the composition ratio of In needs to be made higher. For example, in a case where a MQW that emits green, yellow, orange, or red light is grown, which has a longer wavelength than blue light, this problem becomes prominent.
Meanwhile, there has been a known technique by which an InGaN quantum well layer is grown at a growth temperature of 700° C. with the use of a N2 gas as the carrier gas, the growth temperature is then raised, and H2 gas is added to the carrier gas of N2 at a growth temperature of 900° C., thereby, a GaN barrier layer is grown.